The present invention relates to the field of phased array antenna calibration, and in particular, the calibration of phase and amplitude of the base band signals of the array antenna elements.
Conventional phased array antennae include antenna elements, phase shifters, and attenuator electronics, as well as other elements. The parameters of phase shifters and attenuator electronics vary with temperature and drift with time. As a result, periodic calibration of the phased array antenna is performed to determine phase and amplitude corrections for each antenna element. The present state of the art in the field of phased array calibration includes two techniques. The first technique uses many data points so that phase rotation of the rotating vector yields the location of the maximum or minimum of the signal intensity, and thus the location of the antenna element phase offset. A drawback of this first technique is that many data points must be processed.
A second technique, described in U.S. Pat. No. 5,861,843, uses four specific orthogonal phase states (0, 90, 180, 270 degrees) to perform phased array calibration. A drawback of this second technique is that the calibration measurements must be made at precisely these four orthogonal angles.
In accordance with the principles of the invention, an antenna element of a transmit phased array antenna may be calibrated by first applying a signal having a defined amplitude and phase to each antenna element; maintaining the phase and amplitude of the transmit signal applied to an arbitrarily selected reference element, rotating the transmit phase of the antenna element being calibrated through a sequence of known phase steps while keeping the transmit phase of each of the other antenna elements constant, combining each of the signals from each of the antenna elements at each known phase step of the sequence with corresponding signals from the arbitrarily selected reference element in pairwise fashion to produce a plurality of combined signals, and measuring the combined signals. Thereafter, a phase error and amplitude error for each of the plurality antenna elements is determined based on the plurality of combined signals corresponding to the sequence of known phase steps.
The method of the present invention provides at least two advantages over the prior art. The first advantage is the ability to calibrate the array using sparse, arbitrarily spaced phase angles. For example, the phase states need not be fixed at (0, 90, 180, 270) as in the prior art, but rather may be four points evenly spaced but not these specific, orthogonal states, for example (10, 100, 190, 280). Alternatively, the phase states may be arbitrary and non-uniformly spaced, for example, (10, 90, 190, 300).
The second advantage is the ability to calibrate an antenna element with as few as three phase measurements. For example, the three phase states may be specific and uniformly spaced, for example, (0, 120, 240), they may be arbitrary and uniformly spaced, for example (10, 130, 250) or they may be arbitrary and non-uniformly spaced, for example, (10, 120, 250).
In summary, the method of the present invention permits as few as three point to be used. Further, these three points need not be uniformly spaced or spaced based on an initial phase state of 0 or any other value.
Advantageously, an antenna system may be calibrated more rapidly (than was possible using prior art techniques.) Additionally, calibration may be achieved using a partially corrupt data set by ignoring bad data points by employing smart data selection to ignore obviously corrupt data points.